Manual: pambackground

Pambackground User Manual(1)			   General Commands Manual			Pambackground User Manual(1)

NAME
       pambackground - create a mask of the background area of an image

SYNOPSIS
       pambackground

       [netpbmfile]

       [-verbose]

       Minimum	unique	abbreviations of options are acceptable.  You may use double hyphens instead of single hyphen to de‐
       note options.  You may use white space in place of the equals sign to separate an option name from its value.

DESCRIPTION
       This program is part of Netpbm(1).

       pambackground reads a PNM or PAM image as input.	 It generates as output a PAM image that identifies  the  background
       area of the image (a mask).

       To identify the background, pambackground assumes the image is a foreground image, smaller than the total image size,
       placed  over  a single-color background.	 It assumes that foreground image is solid -- it does not have holes through
       which the background can be seen.  So in specific, pambackground first identifies the background	 color,	 then  finds
       all  contiguous	pixels	of that color in regions touching any edge of the image.  Think of it as starting at each of
       the four edges and moving inward and spreading out as far as possible until it hits  pixels  of	another	 color	(the
       foreground image).

       pambackground  identifies  the  background color as follows: If any 3 corners of the image are the same color, that's
       the background color.  If not, but 2 corners are the same color, the background color is the color of a pair of iden‐
       tically colored corners in this priority order: top, right, left, bottom.  If no two corners have the same color, the
       background color is the color of the upper left corner.

       In a typical photograph, the area that you would consider the background is many shades of a color,  so	to  pamback‐
       ground  it  is multiple colors and pambackground will not meaningfully identify the background of your image.  To use
       pambackground in this case, you might use ppmchange to change all similar colors to a single one first.	For example,
       if the photograph is a building against a blue sky, where nothing remotely sky-blue  appears  in	 the  building,	 you
       could use ppmchange to change all pixels within 20% of "SkyBlue" to SkyBlue, then run pambackground on it.

       You  might  even	 extract the argument for ppmchange from the image in question, using pamgetcolor.  In the foregoing
       example, we knew the background was approximately SkyBlue, but if we didn't we could just get the color	of  the	 top
       left pixel, in a form suitable for the color arguments of ppmchange like this:

	   $ color=$(pamgetcolor 0,0 -infile=/tmp/bodyskl|cut --fields=2 -delim=' ')

       A more convenient means of dealing with a multi-shade background is to use pnmquant to produce a version of the image
       with a very small number of colors.  The background would likely then be all one color.

       If  the	pnmquant and ppmchange methods above do not adequately distinguish foreground colors from background colors,
       you can try a more elaborate method using pnmremap.  If you can manually create a palette with one color to which all
       the background pixels are similar, and other colors to which the foreground pixels are similar, you can use it as in‐
       put to pnmremap to create a smarter version of what you get with the pnmquant or ppmchange methods, so that  pamback‐
       ground is more likely to separate background from foreground as your eye does.

       The  PAM	 that  pambackground creates has a single plane, with a maxval of 1.  The sample value 1 means background; 0
       means foreground.  There is no tuple type.  Some older programs (but none that are part of Netpbm) don't know what  a
       PAM is and expect a mask to be in the form of a PGM or PBM image.  To convert pambackground's output to PBM, use pam‐
       topnm -assume.  To convert to PGM, use pgmtopgm.

       netpbmfile is the file specification of the input file, or - to indicate Standard Input.	 The default is Standard In‐
       put.

       A  common use for a background mask is with pamcomp.  You could replace the entire background (or foreground) of your
       image with something else.

       Another common use is to make an image with the background transparent (in some image format that has  a	 concept  of
       transparency) so that image can be overlaid onto another image later.  Netpbm's converters to image formats that have
       transparency  (e.g.  PNG) let you use the mask that pambackground generates to identify the transparent areas for the
       output.	You can create a PAM image with transparency with pamstack.

       To simply make a mask of all the areas of a specified color, use ppmcolormask.  If you have a unique background color
       (one that doesn't occur in the foreground) and know what it is, this can create a background mask in cases that	pam‐
       background cannot: where there are see-through holes in the foreground image.

OPTIONS
       In  addition  to	 the  options common to all programs based on libnetpbm (most notably -quiet, see  Common Options ),
       pambackground recognizes the following command line option:

       -verbose
	      Tell interesting facts about the process.

EXAMPLES
	   $ pambackground test.ppm | pnminvert >/tmp/bgmask.pgm
	   $ pamcomp -alpha=bgmask.pgm test.ppm wallpaper.ppm >output.ppm

	   $ pnmquant 5 test.pgm | pambackground test.ppm >/tmp/bgmask.pam

SEE ALSO
       ppmcolormask(1), pamcomp(1), ppmchange(1), pnmquant(1), pnmremap(1), pamtopnm(1), pgmtopgm(1),  pamstack(1),  pamget‐
       color(1), pbmmaskd(1), pnm(1), pam(1),

HISTORY
       pambackground was new in Netpbm 10.37 (December 2006).

DOCUMENT SOURCE
       This manual page was generated by the Netpbm tool 'makeman' from HTML source.  The master documentation is at

	      http://netpbm.sourceforge.net/doc/pambackground.html

netpbm documentation				      24 November 2014				Pambackground User Manual(1)